A LASER

Abstract: A pulsed laser comprises a laser rod (6), and is pumped by diode array (3) selected such that with additional components (7, 9) only a maximum of two longitudinal modes are generated between mirror (1) and output coupler (2). By slow Q-switching Q-switch (9) it is hoped to get a single longitudinal mode output pulse. However if on the onset of the relaxation pulse mode beating is detected by detector (28), indicating that two longitudinal modes are present, then the optical path length of the resonator is changed by quarter of one wave length, by applying an electrical signal to a pulse length control means (10), such that one of the modes is suppressed prior to the Q-switch being fully opened. The output may be amplified by amplifier arrangement (24).

Description: A LASER This invention relates to a laser for providing a single longitudinal mode (SLM) output. A laser resonator can sustain waves which have any integral number of half wavelengths. These discrete frequencies of oscillation are known as modes of the resonator. In principle all mode frequencies which lie within the gain bandwidth of the laser medium can oscillate. Usually lasers run on many modes simultaneously, they are multi-transverse and multi-longitudinal modes. Each mode starts from noise (spontaneous emission), a random thermal like process. Initially the mode intensities grow exponentially with time and each mode competes with its neighbours for the available gain. This fierce competition leads to the suppression of weak modes, those that see low net gain or which started with low intensity. The modes nearest the gain centre see the highest gain and reach the saturating intensity first. This line narrowing process can be enhanced by lengthening the build-up time of the oscillation in order to allow more time for mode competition. Etalons and other interferometric techniques are also used to introduce additional losses on unwanted modes. However getting a high gain solid state laser, such as Neodymium: YAG or YLF, to run SLM is still difficult and it has previously been found necessary to ensure the oscillator also runs in a single transverse mode. The present inventors have established that a diode pumped end pumped Neodymium master oscillator will run TEMoo and SLM. This is discussed in the paper: Single Frequency, end pumped Nd: YLF laser excited by a 12mJ diode-laser array; Optical Letters; Dec. 1 1992, Vol. 17, No. 23, the contents of which are hereby incorporated by way of reference. The SLM performance is achieved by slow Q-switching and cavity length control and by pumping the oscillator at low level (lmJ output). The single mode which oscillates is generally that which is closest to the centre of the gain bandwidth (line centre). However this mode has been found to be unstable as the environment changes, and the mode frequency drifts away from line centre until two longitudinal modes occur. The aim of the present mvention is to provide a laser the output of which comprises a single longitudinal mode. According to the present invention there is provided a laser comprising: a resonator; a Q-switch for controlling the level of oscillation within the resonator; means for applying a pump beam to the resonator in an end pumped geometry such that the laser operates in either a single longitudinal mode or in two neighbouring longitudinal modes; means for detecting the presence of more than one longitudinal mode; and means for varying the optical path length of the resonator by a discrete amount in response to the detection of the presence of more than one longitudinal mode, whereby the laser is returned to a single longitudinal mode. The inventors have realised that the instability of the SLM is due to small changes in the optical path length of the resonator. These occur due to physical expansion of the chamber and also due to the change in the refractive index of the optical constituents within the laser, both of which occur because of temperature changes. However the inventors have observed that the laser can be arranged to run such that no more than two longitudinal modes are present, and that these are adjacent modes. Therefore by employing the present invention it is possible, in response to detecting the onset of more than an SLM, to simply vary, or switch, the path length of the resonator by a discrete value in order to return the laser to an SLM. This thereby maintains an SLM without either having to determine the mode or modes initially present, accurately maintain the path length of the resonator, or monitor the response of the laser to the variation made to the path length. The resultant SLM may not be known, but for many applications the exact frequency of the SLM is not important, so long as there is only one SLM present in the output. This is particularly advantageous where the laser is a pulsed laser. When two modes are present they are adjacent modes and nearest neighbour mode beating occurs. Advantageously the presence of more than one longitudinal mode is detected by detecting this mode beating. Mode beating will only occur when the resonator optical path length has changed sufficiently for two adjacent longitudinal modes to be nearly symmetrically placed about the line centre, otherwise the dominant mode prevails to the detriment of the other. It is therefore preferable that the means for varying the optical path length switches the optical path length of the resonator by substantially an odd integral multiple of one quarter of the wavelength at which the laser is operating, (ie l/4λ, 3/4λ, 5/4λ , ...), as this will move one of the modes back close to line centre. It will be appreciated that it is irrelevant whether the path length is shortened or extended for in either case one of the modes will be brought onto the line centre such that only an SLM is present. Therefore the means for varying the path length need only have two states. The means for varying the optical path length can conveniently comprise a piezo electric transducer driving a mirror defining the resonator. However, piezo electric transducers are relatively slow and it has been found preferable to have means for varying the optical path length comprising an element in the optical path, the refractive index of the element being altered by the application of a voltage. This voltage can have one of two values, one of which may be zero, which causes the path length through the block of material to alter by a quarter of one wavelength. The switch thus only has two states. Preferably a laser in accordance with the invention further comprises control means for the Q-switch and the means for varying the optical path length, wherein the control means partially opens the Q-switch, (which partial opening can be a progressive action), in response to the application of a pump pulse to the resonator until a relaxation pulse appears which the control means monitors. If only an SLM is detected (by the absence of beats) the control means causes the the Q-switch to be opened further. If more than one single longitudinal mode is detected (presence of beats) the control means causes the means for varying the optical path length to alter the optical path length. By partially opening the Q-switch of the cavity during the pump pulse, low level oscillation builds up over the pump duration, resulting in a low intensity relaxation pulse. If the resonator length is correct, mode competition during the many round trips results in the selection of an SLM, which is detected by the absence of any mode beating on the relaxation pulse. The Q-switch is then fully opened, resulting in an SLM giant pulse. If however mode beating is detected, due to changes in the resonator length having caused the mode frequency to drift away from line centre, then instead of opening the Q-switch fully the path length is changed, preferably by a quarter of one wavelength. The next relaxation pulse, which typically follows the first by about 10 us, will then comprise an SLM and can be fully Q-switched. It is advantageous wherein if two longitudinal modes are present the path length is changed between successive relaxation pulses such that the next output pulse from the laser comprises an SLM. Preferably the laser is end pumped and advantageously this is achieved by diode pumping. Diode end pumping the laser can be arranged to limit its oscillation to single transverse mode and either one or two longitudinal modes at any one time. Another advantage of the present invention is that the beat frequency to be monitored is that of the nearest neighbour modes only, which in a typical application would be in the range of 500 to 800 MHz, which is a relatively easy range to monitor. A preferred lasing medium is Neodymium doped Yttrium lithium fluoride (Nd:YLF). One embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, of which: Figure 1 A is a schematic side elevation of the components of a laser in accordance with the present invention; Figure IB is a plan view of the components of figure 1 A; Figure 2 is a perspective view of a laser in accordance with the present invention incorporating the components illustrated in figures 1A and IB; and Figure 3 is a perspective view of the amplifier 24 of figure 2. Referring to Figures 1 A and IB, the resonant cavity of the laser is defined by mirror 1 and concave coupler 2. Light from diode array 3 is focused by cylindrical lens 4 and aspheric lens 5 onto a laser medium which comprises a Nd:YLF rod 6 within the resonator cavity. Light within the cavity resonates through an anamorphic prism pair 7 and polariser 8, Q-switch 9 and electro-optic path length controller 10. A practical implementation of the components illustrated in Figures 1A and IB is illustrated in Figure 2, in which like components to those in Figures 1A and IB have the same reference numerals. Referring to Figure 2, the components are seen mounted on a flat aluminium chassis 20, the approximate dimensions of which are 350 mm x 150 mm x 25 mm. In addition to the components referred to above with reference to Figures 1A and IB, there are additionally mounted on the chassis a polariser 21, a folding prism 22, a wave plate 23, amplifier 24 and a Porro prism 25, and under the chassis, (all of which are shown detached from the chassis for clarity), electro-optic length control electronics 26, Q-switch modulator 27 and a mode beat monitor 28. The mode beat monitor 28 has attached to its upper surface a metal conduit 29 whic screens an optical path between a detector element located in the mode beat detector 2 and a pick-off in the optical path (not shown). In operation a pump source is provided by diode array 3 for the Nd: YLF oscillator. Th radiation from the diode is temperature tuned to the strong 797nm absorption line of th Nd:YLF rod, and is used in an end pumped configuration. The Nd:YLF rod i longitudinally pumped by a diode laser array which is mode matched by means of th diode coupling and inter-cavity mode shaping optics 4, 5. The Nd:YLF rod is 1% doped with Neodymium and the wavelength of operation i 1.053 um. The mirror 1 is anti-reflective at 800 nm and highly reflective at 1053 nra permitting the pump beam to pass to the rod 6. The rod 6 is orientated such that its axis is collinear with the resonator axis, reducing thermally induced birefringence to negligible amount and allowing pumping along the higher absorption C axis. The resonator cavity formed by mirror 1 and concave output coupler 2 is approximatel 20 cm long and contains polariser 8 which is dielectrically coated and airspace providing polarisation and wavelength selection for the resonator. The polariser 8 als provides the necessary contrast with the electro-optic lithium niobate (LiNbO3) crystal Q-switch 9. The resonator operates in an elliptical TEMoo mode, and the curvature of the concave output coupler is selected to ensure only the lowest order resonator mode in the vertical plane has sufficient gain to lase, while this mode is expanded in the horizontal plane so as to fill the pumped volume. Magnification is accomplished using anamorphic prism pair 7 which magnifies the resonator mode in the horizontal plane by a factor of three. In operation the Q-switch modulator 27 applies a voltage to the Q-switch 9 during the pump pulse to the diode 3, such that the Q-switch is opened slightly allowing low level oscillation to take place over the pump duration, the pre-lase phase. Under the right conditions mode competition during the many round trips results in the selection of a single axial, or longitudinal, mode. The existence of this single mode is detected by the absence of any mode beating sensed by mode beat monitor 28 on the ensuing relaxation pulse. If no mode beating is present the Q-switch is fully opened by Q-switch modulator 27 and a single mode giant pulse is output through the concave output coupler 2. If the optical path length within the resonator varies, then the mode frequency drifts away from the line centre. This is not a problem until two adjacent longitudinal modes are nearly symmetrically placed about the line centre, at which point strong mode beating is observed. If this occurs, then when the Q-switch is slightly opened, the mode beat monitor detect the mode beating at the onset of the relaxation pulse. The electro-optical path length control electronics 26 then alters the voltage applied to the electro-optic path length control 10 by the amount required to give a change in the path length through the lithium niobate of one quarter of a wavelength. The path length change required is of the order of 0.25 micron. This change in wavelength puts one of the adjacent modes back close to line centre. Thi change of path length is accomplished between oscillations in the train of relaxatio pulses such that SLM operation normally occurs within two relaxation oscillations suc that the first allowed giant pulse of the laser is normally SLM. To increase the output power from the concave output coupler 2, the output is passe through polariser 21, folding prism 22, and wave plate 23 such that it makes double pas through the amplifier cavity 24 via Porro prism 25 before being output at 29. The amplifier cavity 24 is illustrated in more detail in Figure 3 and comprises coolan input pipes 30 and coolant output pipes 31 for cooling the diode arrays 32 and end plate 33. In the middle of the six diode arrays 32 is located the amplifier cavity which consists of a Nd: YLF rod 34 which is located within the six diode array stacks 32 and glass flow tube 35. Coolant passes in one end plate 33 along the flow tube 35 and out the other end plate 33. Suitable components for the amplifier are: for the diode arrays 32, Spectra Diode Laboratories type SDL3230ZL diode laser array; and for the laser rod a 1% at.Wt.Nd doped YLF lasing medium. It will be realised that the apparatus illustrated in the attached figures will require conventional support equipment, which for clarity has not been illustrated. This support equipment includes diode array drivers, a liquid cooling unit for the amplifier and a Peltier controller for Peltier coolers (not shown), which are attached to the body of diode arrays 3, 32 and the control circuit for the cavity length adjustment. A DC power supply unit will also be required to generate the DC supply used for the mode beat detector, Q-switch modulator and electro-optic length control module and also for providing low voltage supplies to the control units. The attached figures illustrate one embodiment of the invention only. It will be readily appreciated that alternative geometries, including ring lasing geometries can be constructed in accordance with the present invention.

Claims: CLAIMS 1. A laser comprising: a resonator; a Q-switch for controlling the level of oscillation within the resonator; means for applying a pump beam to the resonator in an end pumped geometry such that the laser operates in either a single longitudinal mode or in two neighbouring longitudinal modes; means for detecting the presence of more than one longitudinal mode; and means for varying the optical path length of the resonator by a discrete amount in response to the detection of the presence of more than one longitudinal mode, whereby the laser is returned to a single longitudinal mode. 2. A laser as claimed in Claim 1 wherein the presence of more than one longitudinal mode is detected by means for detecting mode beating. 3. A laser as claimed in claim 1 or 2 wherein the means for varying the optical path length switches the optical path length of the resonator by substantially an odd integral multiple of one quarter of the wavelength at which the laser is operating. 4. A laser as claimed in claim 2 or 3 wherein the means for varying the optical path length comprises an element in the optical path, the refractive index of the element being altered by application of a voltage. 5. A laser as claimed in claim 4 wherein the voltage applied to the means for varying the optical the path length has one of two values. 6. A laser as claimed in claim 1, 2 or 3 wherein the means for varying the optical path length comprises a piezo- electric transducer which drives a mirror of the resonator. 7. A laser as claimed in any preceding claim which is a pulsed laser. 8. A laser as claimed in claim 7 further comprising control means for the Q-switch and the means for varying the optical path length, wherein the control means partially opens the Q-switch in response to the application of a pump pulse to the resonator until a relaxation pulse appears, which the control means monitors, if only a single longitudinal mode is detected the control means causes the Q-switch to be opened further, and if more than one longitudinal mode is detected the control means causes the means for varying the optical path length to alter the optical path length. 9. A laser as claimed in any preceding claim wherein if two longitudinal modes are present the path length is changed between successive relaxation pulses such that the next output pulse from the laser comprises a single longitudinal mode. 10. A laser as claimed in any preceding claim which is diode pumped. 11. A laser as claimed in any preceding claim in which the lasing medium is Neodymium doped Yttrium Lithium Fluoride (Nd:YLF).

Inventors: WHITTLEY STEWART TREVOR, WHITTLEY, STEWART, TREVOR
Applicants: GEC MARCONI AVIONICS HOLDINGS, WHITTLEY STEWART TREVOR, GEC-MARCONI AVIONICS (HOLDINGS) LIMITED, WHITTLEY, STEWART, TREVOR
Application year: 1994
Main classification: H01S3
IPC codes: H01S3